1. Field of the Invention
The present invention relates to touch cameras used in imaging and decoding information-containing symbols, and more particularly, to an improved method and apparatus for capturing machine-readable bar and matrix code symbols from various surfaces, including highly reflective, textured, or curved surfaces.
2. Description of the Related Art
Many industries, including those involving component assembly, processing, or inventorying, use an identification system in which the products are marked with a coded symbol containing information about the products, such as bar code or matrix symbols.
A number of different readers and laser scanning devices have been developed to decode the symbol pattern to a multiple numeric or alpha/numeric representation. These "optical scanners" are available in a variety of configurations, and are either incorporated into fixed station apparatus or portable apparatus.
The portability of an optical scanner enhances the ability to inventory products on shelves and to track portable items such as files or small equipment. A number of these portable scanning devices incorporate laser diodes, which allow the user to scan the coded symbols at various distances from the surface on which the coded symbol is located. The principal disadvantage of such scanners is that they are unable to read coded symbols which have been placed on highly reflective, textured, or patterned surfaces. A second disadvantage of such scanners is their high cost of manufacture.
Another type of coded symbol scanner, which is embodied in a portable apparatus, uses light emitting diodes (LEDs) as a light source and a charged coupled device (CCD) as a detector. This class of coded symbol scanners is generally known as CCD scanners. While they have the advantage of being less expensive to manufacture, they also require that the scanner be in contact with the surface on which the symbol is located.
However, the imaging capabilities of such cameras, known as "touch or contact" cameras, are typically adversely affected by specular reflection emitted from highly reflective, smooth surfaces, or shadowing produced by minor surface defects, and patterns or surface textures produced by machining.
The solid state image sensor used in CCD touch cameras is a matrix array of light sensitive monolithic silicon chips ("pixels") which absorb photons. A typical 512.times.512 matrix sensor array contains 262,144 chips. During operation, the solid-state image sensor converts incident light to electric charge which is integrated and stored until the time of readout. The integrated charge is directly proportional to the intensity of the light on the sensing elements. Readout is initiated by a periodic start or transfer pulse. The charge information is then sequentially read out at a rate determined by clock pulses applied to the image sensor. The output is a discrete time analog representation of the spatial distribution of light intensity across the array.
The primary disadvantage of using CCD cameras in imaging ("capturing") machine-readable symbols is a phenomenon known as "blooming", a condition which can severely degrade the performance of the image sensor and can cause problems with the camera circuitry. Blooming occurs when an excessive number of light generating electrons are produced. The result is that bright parts of the image smear and spread out into the surrounding darker areas of the image creating false responses from the pixels in those areas. The effect appears similar to the petals of a flower blooming out from a bud; hence, the name "blooming". Often "blooming" occurs as an unavoidable anomaly to normally expected conditions, such as specular reflections or other transient high intensity light.
The total charge in any given pixel is the result of photon absorption which creates electrons that accumulate over the integration period; thus, the total photo-electron generation ("photocurrent"). The brighter the light intensity, the higher the photocurrent. Excessive charge can be created by either excessive illumination or too lengthy an integration time. The excessive charge cannot be completely discharged within the average time a human can hold such cameras still (normally 1/60 second), so it leaks past the transfer gate of the CCD and appears as a vertical stripe on a video monitor. An even greater intensity light produces a charge that not only leaks past the transfer gate, but also past the channel stoppers of the other pictures, thus causing a washout area in the video image. This condition is technically known as depth overflow.
These problems can be overcome in most fixed station camera applications by controlling the external lighting conditions using special equipment, as for example, portable studio lights, filters and reflectors. This equipment, however, is not practical for use in the field where lighting conditions change on a continual basis.
To overcome the specular reflection problem, field use cameras are best operated in the "touch" mode, where the optical windows of the cameras are pressed against a substrate's surface atop the coded symbol to be imaged in order to block out all incident light. The surface is then illuminated with a built-in light source which emits light having an optimum intensity, angle and wavelength. Cameras of this type are extremely effective for imaging high quality paper labels applied to flat surfaces, but they are ineffective for acquiring images from rough or reflective materials or curved surfaces.
Symbol reading cameras which are known are disclosed in the U.S. Pat. Nos. to Beyor (U.S. Pat. No. 4,742,220), August (U.S. Pat. No. 3,961,198), Katana et al. (U.S. Pat. No. 4,743,773), Hara et al. (U.S. Pat. No. 4,818,847), Swartz (U.S. Pat. No. 4,825,057), Matusima et al. (U.S. Pat. No. 4,900,907) , Baumberger (U.S. Pat. No. 4,908,500), Dolash (U.S. Pat. No. 4,983,817), Roustaei (U.S. Pat. Nos. 5,291,009 and 5,354,977) , and Powell et al. (U.S. Pat. No. 5,350,909).
None of these cameras, however, are effective in a "touch" mode to permit symbol capture from roughened, highly reflective, or curved surfaces.